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Offline xavierTopic starter
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« on: May 10, 2013, 03:12:59 am »
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Hi guys

Here is an analysis that I have been doing in order to better understand the Op-Amp and of course I'm using a metal detector or rather a part there off to do my testing. 
I have an ordinary Colpitts oscillator connected to a D shape TX coil and a D shape RX coil overlapping the TX coil (See photo one)  I have the oscilloscope connected on the RX coil, the frequency can be seen in photo two. (So far so good) Here is where the problem starts I have the RX coil going to a differential amp at the out put of the amp I must get a zero out put until either the non-inverting is greater than the inverting or vice versa. So when I pass a piece of metal over the coil I should be getting a high or a low at the output pin of my differential amplifier but I'm not getting anything, zip, nada no matter how low I set the voltage on the scope. But why?       

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Offline GoldDigger1950
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« Reply #1 on: May 10, 2013, 06:40:09 am »
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There are two modes of basic Op-Amp operation. Voltage mode and current mode. If your circuit is configured in the current mode, you will see no voltage change on your output. To see a change, put a medium sized resistor in series with your output. Say around 100k or so. Attach your ground lead to either side of that resistor and the probe side to the other. See what develops then.

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« Reply #2 on: May 10, 2013, 07:34:55 am »
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Most probably your coil is not balanced well enough to notice anything. Unless you achieve balance below 10mV at coil, any metal target will produce negligible difference.

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Offline xavierTopic starter
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« Reply #3 on: May 10, 2013, 12:10:28 pm »
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Quote:Posted by GoldDigger1950
To see a change, put a medium sized resistor in series with your output. Say around 100k or so. Attach your ground lead to either side of that resistor and the probe side to the other. See what develops then.

Nothing all that I'm getting is noise coming from the TX coil. 

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« Reply #4 on: May 10, 2013, 12:36:54 pm »
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In Voltage Mode the Zero Volts setting can effect the Trigger point as well.   You have it working sorta like a Comparator at that point.


Another thing you can try is to change the Resistor on the Input or feed back loop.    Raise the Gain.

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« Last Edit: May 10, 2013, 12:39:13 pm by homefire »
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Offline xavierTopic starter
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« Reply #5 on: May 10, 2013, 12:50:13 pm »
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The thing is that I'm using an existing design and the coil was made for it too, I must admit that I'm starting to get frustrated with this  (#@% how difficult can it be to get a freaking amp to work?)  LOL I'll probably be kicking my self when I find the problem.

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« Reply #6 on: May 10, 2013, 01:02:36 pm »
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All chips of the same numbers are not created equal!

Try some others.

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Offline xavierTopic starter
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« Reply #7 on: May 10, 2013, 01:25:09 pm »
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Quote:Posted by homefire
Try some others.

Did that too............I'm missing something here that's for sure I'm using +8v and -5v as in the original design so it cant be the voltage.

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« Reply #8 on: May 10, 2013, 02:27:54 pm »
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PM the Set up to GD and Me.     See what we see.    OK, I'm not GD but I do know some things.

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« Reply #9 on: May 10, 2013, 03:12:11 pm »
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OK will do that

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